1. Technical Field
Embodiments of the invention relate to semiconductor devices used in automobile internal combustion engine ignition devices, as well as other devices.
2. Related Art
FIG. 17 shows an example of a configuration of a heretofore known general internal combustion engine ignition semiconductor device using an insulated gate bipolar transistor (hereafter referred to as an IGBT) as a power semiconductor element. See, for example, Japanese Patent Application No. JP-A-2012-36848.
An ignition device shown in FIG. 17 is configured of an engine control unit (hereafter referred to as an ECU) 1, a semiconductor device 5, and an ignition unit 7.
The ignition device shown in FIG. 17 is such that, on an abnormal condition being detected, a self-interrupt signal Vsd is emitted from a self-interrupt signal source 10, a self-interrupting circuit 33 operates, and a collector current Ic of an IGBT Tr2 is interrupted. An abnormal condition is a condition such that there is a danger of damage such as burning occurring in an ignition coil L or the semiconductor device 5, such condition being, for example, a turn-on signal output from the ECU 1 being longer than a predetermined time (for example, 10 ms or longer), or the temperature of the semiconductor device 5 being higher than a specified value (for example, 180° C. or higher).
However, when the collector current Ic is abruptly interrupted using this kind of current control function or self-interrupt function, fluctuation is caused in the collector current Ic, and there is a problem in that erroneous ignition of a spark plug 4 is caused, and the engine is damaged.
Technology whereby the collector current Ic is gently reduced being known as a countermeasure to erroneous ignition caused by the fluctuation of the collector current Ic, a method whereby a soft shut-off circuit is provided, and a gentle reduction time set, is disclosed in Japanese Patent Application No. JP-A-2008-45514. Also, a method whereby an integrated circuit formed of a diode and capacitor is provided, and a collector current Ic gentle reduction time set, is disclosed in Japanese Patent Application No. JP-A-2006-37822.
Meanwhile, a current control circuit 6 shown in FIG. 17 includes a self-interrupting circuit 33 shown in FIG. 18. The self-interrupting circuit 33 is configured of a bias circuit wherein a DepMOSFET (Depression Metal-Oxide Semiconductor Field-Effect Transistor, hereafter referred to as a DepMOS) Tr7 and a MOSFET (Metal-Oxide Semiconductor Field-Effect Transistor, hereafter referred to as a MOS) Tr8 are connected in series with a common gate, a MOS Tr9 configuring a current mirror circuit with the MOS Tr8, a MOS Tr4 connected in series with the MOS Tr9, an inverter NOT1 connected to the gate of the MOS Tr4, and a capacitor C1 connected in parallel to the MOS Tr9.
The MOS Tr4 is controlled on and off by the self-interrupt signal Vsd, being in an on-state at a time of no al operation and an off-state at a time of abnormal operation. Also, by the on-state resistance of the MOS Tr4 being set sufficiently low in comparison with the on-state resistance of the MOS Tr9, the capacitor C1 is charged at a time of normal operation, and a reference voltage Vref is output as it is, while at a time of abnormal operation, the output voltage gradually drops from Vref to 0V by the capacitor C1 being discharged via the MOS Tr9.
An operational amplifier OP1 detects the difference between a voltage (hereafter referred to as a sense voltage) Vsns across a sense resistor R1 and the reference voltage Vref, wherein a target value of the collector current (the current caused to flow through a primary coil L1) Ic is converted into voltage, the two voltages having been level shifted via level shift circuits 9 and 15 respectively. The on-state resistance of the MOS Tr3 is controlled by the gate voltage of the MOS Tr3 being controlled in accordance with the result of the detection.
The ignition semiconductor device shown in FIG. 17 is such that, in order to reduce the capacitance C1 without changing a temporal amount of change (hereafter called the inclination) dlc/dt in the collector current, it is sufficient to reduce a drain current Id of the MOS Tr9. However, as the drain current Id is an extremely low current in the order of nA, the more the drain current Id is reduced, the more difficult it becomes to maintain a constant current. Consequently, reducing the drain current Id is not desirable. Also, as erroneous ignition occurs when the inclination dlc/dt of the collector current is too large, while the IGBT or ignition coil L is burned when the inclination dlc/dt is too small, there is a demand for a high precision drain current Id control.
Thus, as described in the related art, there is a limit to the reduction of capacitance of the self-interrupting circuit capacitor and the accuracy of current control, as well as other shortcomings.